Chess Opening Theory/1. d4/1...d5/2. c4/2...c6

Originally considered a less-orthodox defence in the Queen's Gambit, this opening has long stood as an entire opening system in its own right for decades.

The idea behind this defence is straightforward: instead of locking in the light squared bishop on c8, why not support the center with the c-pawn instead? Black tends to be more active in this variation than the QGD.

White's main lines:

3. Nf3 - The main line. As with the QGD, white makes a useful developing move while maintaining some flexibility.

3. Nc3 - Sometimes provocative, this move may lead to variations where black overextends by trying to hold on to the pawn on c4.

3. cxd5 - The exchange variation. This relieves the central tension early on and tends to allow black equality. One of the main drawbacks of having the Slav as a main opening repetoire is that it is hard for Black (or White) to win in the exchange variation. According to chess365, 83% of Masters games ended in a draw after this move. Even so, it is important for both sides to know.